1. Field of the Invention
The present invention relates to a control system for charging which enables efficient charging of rechargeable batteries, a control system for charging which enables accurate charging of the rechargeable batteries, and an electronic apparatus which enables measurement of the charging/discharging current of the rechargeable batteries by a simple configuration.
2. Description of the Related Art
A portable electronic apparatus such as a notebook computer carries batteries as the power supply for the apparatus. Generally, rechargeable batteries (rechargeable batteries) such as NiCd, NiMH (nickel metal hydride), or Li+ batteries are mounted due to the operating costs of the apparatus, the desire for an instantaneously dischargable current capacity, etc. Also, there are many examples in which a charger is built in so as to enable easy charging of the rechargeable batteries mounted inside the apparatus by just connecting an AC adapter etc. to the apparatus.
In such a portable electronic apparatus, usually the internal rechargeable batteries are used as the power supply of the apparatus, but when operating the apparatus on a desk etc., the apparatus is also operated while drawing power from an external power source through an AC adapter.
If the power which can be supplied from the AC adapter connected to the apparatus is sufficiently larger than the maximum power used by the apparatus and the maximum power necessary for the charging of the rechargeable batteries, it is possible to simultaneously operate the apparatus and charge the internal rechargeable batteries. However, where the capability of the AC adapter is smaller than this, supply of power for both of the operation of the apparatus and the charging of the internal batteries becomes impossible. Therefore, just one of them is performed in accordance with the status of use of the apparatus. AC adapters actually used for such apparatuses are limited in the amount of power which they can supply due to cost and size factors. In general, it is a rare that both operations are simultaneously carried out.
Usually, so as to minimize the cost and size of the AC adapter, generally the capability of the AC adapter is set to the larger of the maximum power necessary for the charging of the internal rechargeable batteries and the maximum power to be used by the apparatus. Also, in an apparatus designed to operate on the batteries, generally the power for charging the internal rechargeable batteries is larger than the maximum power consumption of the apparatus. This is because if the reverse were true, then the time during which the apparatus could be operated on the batteries would become shorter than the time required for charging the batteries. This would not be practical for a commercial apparatus.
In view of this situation, up until now the method has been adopted of having the charger which is mounted inside the apparatus constantly monitor the status of the apparatus and charge the rechargeable batteries when the power switch of the apparatus is turned OFF, stop the charging to the rechargeable batteries when the power switch of the apparatus has been turned ON, and restart the charging when the power switch of the apparatus is returned to the OFF position. Namely, in this configuration, the charging to the rechargeable batteries was carried out when the apparatus was not being operated and the charging to the rechargeable batteries was stopped when the apparatus was being operated.
With this setup, however, when there is excess capability of the AC adapter, efficient charging cannot be performed. Therefore, recently, the method has been adopted wherein, where the capability of the AC adapter is larger by a certain extent than the maximum power to be used by the apparatus, when the power switch of the apparatus is turned ON, the current for charging the rechargeable batteries is lowered and the charging is continued, and when the power switch of the apparatus is turned OFF, the charging is carried out by the original charging current. Namely, in this configuration, when the apparatus is not being operated, a large charging current is generated for charging the rechargeable batteries, while when the apparatus is being operated, a small charging current is generated for charging the rechargeable batteries.
In a conventional charging system having such a configuration, as will be explained in detail later with reference to the drawings, usually the completion of the charging is detected by using the “history control technique”, that is, monitoring the elapse of time, and “maximum temperature control technique”, that is, monitoring the maximum temperature. Between these two, the most widely used technique has been the detection of the completion of charging by monitoring the elapse of time from the start of the charging.
Further, there are sometimes demands for measurement of the discharging current of the rechargeable batteries as well in such charging processing. Conventionally, such demands have been met by the technique of preparing, separately from a first sense resistor for detecting the charging current of the rechargeable batteries, a second sense resistor for detecting the charging current of the rechargeable batteries and detecting the discharging current of the rechargeable batteries using this second sense resistor.
However, there was a problem in that efficient charging was not possible when, as in the related art, a larger of two levels of charging current was generated for charging the rechargeable batteries when the power switch of the apparatus is turned OFF and a smaller of two levels of charging current was generated for charging the rechargeable batteries when the power switch of the apparatus is turned ON.
Namely, according to this related art, the magnitude of the charging current must be set irrespective of the magnitude of the current consumed by the apparatus, so the charging current when the power switch of the apparatus is turned ON must be set to the lowest level, that is, the one for when the current consumption becomes the greatest. Due to this, there was a problem in that the charging was not performed using the capability of the external power source such as the AC adapter to the fullest extent.
Also, when the power switch of the apparatus was left in the ON position, irrespective of whether or not the apparatus was actually being operated, the rechargeable batteries were charged with a small charging current. Therefore, there was a problem that efficient charging was not performed.
If a method of detecting the completion of charging of the rechargeable batteries by monitoring the elapse of time from the start of the charging is adopted as in the related art, where the charging current generated by the charger dynamically changes, there was a problem that the completion of charging of the rechargeable batteries could not be accurately detected.
Further, if the method is adopted of preparing a sense resistor for detecting the charging current of the rechargeable batteries and a separate sense resistor for detecting the discharging current of the rechargeable batteries as in the related art, there was a problem that two sense resistors became necessary and therefore the charging/discharging current of the rechargeable batteries could not be measured by a simple configuration.